Unless you have access to a computer that can recalibrate your speedometer by reprogramming the ECU, I'd suggest taking it to a speedo shop and have them do it.
Second alternative would be to use a GPS to monitor speed or at least give you the 'real' speed to compare to the dash speedo. This will also make your odometer read less miles than actually travelled.
Third alternative is go to Walmart and buy a
bicycle speedometer for $10.49. Use a string to measure circumference of the tire, mount the bike speedo magnet on the inside rim, fasten the detector inline with the magnet, run the wire into the dash and put the speedometer read out in sight of the driver. Once you enter the circumference of the tire, you'll have a speedometer that will give you readings to .1 mph. (I did this in a car. The original speedometer took a reading from the transmission, an internal plastic gear broke. It would require the R/R of the transmission to fix the plastic gear, so I optioned for the bike speedo. It worked great!)
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